The present invention pertains to a system for compressing a conveyed line of paper or paperboard sheets into a shingle and, more particularly, to such a system utilizing a dual plenum vacuum shingling device. The system may also include a shingle separation subsystem.
Vacuum shingling is well known and well developed in the art of handling sheets of paper and paperboard. When sheets of paper or paperboard are cut to length for further downstream conversion, they are usually delivered from a knife or other cutoff device as a high speed line of closely spaced sheets, often moving at a speed of 1,000 feet per second (about 300 meters per second) or more. In order to compress the line of sheets to facilitate handling, as for example for forming stacks of sheets, the line of sheets is formed into a shingle which continues to advance at a much reduced speed. In order to form a shingle, the sheets must be slowed considerably and handled in a manner such that the lead edge of each following sheet is made to overrun the tail edge of the sheet immediately preceding it. This may require the sheets to be slowed on a shingling conveyor to a speed that is only 20% of incoming line speed or less.
Because of wide variations in line speed at which the sheets are fed, the percent shingle (overlap) required, sheet length and basis weight of the paper or paperboard sheets, many different ways have been developed for shingling and for controlling sheets in the shingling process. Another complication is introduced when sheets are preprinted or finished on the exposed top sides such that contact of the sheets with overhead snubber wheels, brushes or the like is undesirable or impossible. In such cases, vacuum shingling by which the sheets are captured and slowed from line speed by applying a vacuum to the undersides of the sheets is a common practice.
Nevertheless, it would be desirable to have a vacuum shingling system that would be adaptable to handle a wider range of sheet sizes and basis weights, over a wide range of delivery line speeds and shingle overlap and, in particular, with a system that would not include devices that rub and could scuff finished upper sheet surfaces.